Our research is aimed toward an understanding of the biochemical and functional characteristics of MHC-peptide and [MHC/peptide]-T Cell Receptor (TcR) interactions. These characterizations underlie a fundamental understanding of immune responses to a variety of antigens and the rational development of peptide-based immunotherapies. We are currently studying how the TcR of a murine CTL clone recognizes two different peptide/MHC class I complexes, since the ability of TcR to engage dissimilar epitopes may play an important role in T cell development and peripheral function. This CTL was originally isolated as a xenoreactive clone specific for the human class I molecule HLA-A2.1 complexed with an endogenous human peptide. We were able to identify a synthetic peptide which forms a complex with H-2 Db that is also recognized by this CTL clone. This finding suggests that the TcR of this CTL clone "sees" these two epitopes in a non-identical manner, making both shared and complex- specific contacts with each peptide/class I combination. X-ray crystal structures of these two complexes have been obtained, and structural insights into their topologies suggest that the critical contacts which contribute to TcR-ligand binding energy may be formed at several locations along the molecular interface. A peptide derived from protein Melan A/MART1 (AAGIGILTV, residues 27-35) is presented by class I HLA-A2-1 molecules on melanoma cells, and CTL reactive to this self epitope are frequently recovered from tumor- infiltrating lymphocytes (TIL) of HLA-A2.1 melanoma patients. We have explored the basis for the predisposition of melanoma patients to respond to this epitope and found that sequences similar to AAGIGILTV occur frequently among proteins from viral, bacterial and endogenous sources. Several of these sequences are capable of sensitizing target cells for lysis by patient CTL. Since some self proteins contain MART127-35 sequences, we have also focused our attention on the possibility that peripheral CTL encounters with self peptides might play a role in shaping the response to MART127-35 on tumors. Several peptides were found to be partial agonists or antagonists of MART127-35-reactive CTL clones derived from TIL. In sequential exposure experiments, some antagonists selectively abrogated the secretion of IL-2 elicited by subsequent agonist exposure, leaving cytolysis and IFN gamma release intact. These peptides also gave rise to altered phosphorylation patterns of T cell signaling components, characteristics of non-agonist TcR ligands. These data raise the possibility that self peptides contribute to the maintenance of MART127-35-reactive CTL while having a net negative influence on their functional efficacy, and are consistent with emerging models of T cell function which suggest that self peptides may actively shape the physiology of peripheral T cells.